56 



MANUAL OF BOTANY 



Fig. 804. 



or having a pair of cilia at one end, which is usually pointed, 

 giving them a pear-shaped appearance. They are not produced 

 in specially differentiated, gonidangia ; apparently any cell of the 

 thallus can give rise to them. 



The sub-class has been very variously subdivided, and at 

 present it is difficult to give a very minute classification. 

 Several types of structure may be briefly described. 



Protococcace,^. — These are the simplest of the sub-class ; 

 the plants consist of single cells or of colonies of cells united 

 by a common mucilaginous cell-wall. The^^ multiply by cell- 

 division, or by formation of zoospores, or by the conjugation 

 of two free-swimming gametes. In the latter case the zygote 

 represents the sporophyte and gives rise to two zoospores on 



germination. 



YoLvoccACE.E. — Of these the most 

 remarkable plant is Volvox itself. It 

 consists of a hollow sphere formed of a 

 single layer of cells, each of which is 

 fm-nished with a pair of cilia. By 

 the movement of these the plant 

 rotates rapidly and makes its way 

 through the water. Some of the cells 

 of the sphere produce a number of 

 orange-coloured ciliated antherozoids ; 

 others represent oogonia, and each 

 gives rise to a single oosphere. The 

 latter is fertilised in situ and divides 

 into a number of cells, wiiich become 

 arranged into a hollow sphere like 

 the parent. Certain other cells of the 

 colony, which may be taken to represent gonidia, can divide 

 similarly in situ, and produce within their cell-wall a miniature 

 Volvox. Several of these may often be seen in the interior of 

 the hollow sphere of the parent, into which they are discharged 

 from the gonidangial cell. They only escape on the death of 

 the parent. Other genera of this group are Pandorina and 

 Eudorina; the coenobium of the former is a solid sphere, that 

 of the latter a hollow one. Pandorina reproduces itself by 

 gametes which are alike, Eudorina by antherozoids and 

 oospheres. The zygote of Pandorina represents the sporophyte 

 and produces zoospores. That of Eudorina is like that of 

 Volvox. Other genera produce colonies or coenobia which are 

 fl^ittened instead of spherical, 



Fig. 804. Branclieil cell (Bo 

 trydium granidatum). 



